A reciprocating compression-ignition internal combustion engine, otherwise known as a diesel engine, is well known. In the diesel engine, air is drawn into the combustion chamber during the induction stroke of the piston, and is compressed to a high compression ratio. As a result of this compression, the air in the combustion chamber is heated to a temperature higher than the ignition temperature of the diesel fuel. Diesel fuel is then injected into the combustion chamber. Because of the prevailing high temperature, the fuel ignites spontaneously. However, combustion does not take place immediately when the fuel particles enter the combustion chamber. The fuel droplets must first mix intimately with the air in the combustion chamber, and then be heated and vaporized before they can burn. The time lapse between fuel injection and ignition during which this process occurs is known as the "ignition delay" or "ignition lag".
Diesel engines, like spark-ignition engines, operate on either the four-stroke or the two-stroke principle. In a direct injection type diesel engine, the fuel is injected directly into the combustion chamber, rather than into a swirl chamber or a precombustion chamber, as with indirect injection diesel engines.
The ignitability of a diesel fuel is rated by its centane number (C.N.). The centane number of a diesel fuel is the percentage by volume of normal centane in a mixture of centane and alphamethylnaphthalene which matches the unknown fuel in ignition quality when compared in an ASTM-CFR diesel engine by a specified procedure. The C.N. scale ranges from 0 to 100 C.N. for fuels equivalent in ignition quality to alphamethylnaphthalene and centane, respectively. The higher the centane number of a fuel, the greater its ignitability in a diesel engine.
A two-phase combustion process takes place in the diesel engine. The first phase, called "premix combustion", is the spontaneous burning of the fuel first injected into the hot compressed air. As additional fuel is injected, it is ignited by the flame resulting from the premix combustion phase. This second phase is called "diffusion combustion".
Modern environmental concerns require that diesel engines operate with minimum fuel consumption, minimum noise levels, and minimum NO.sub.x and particulate emissions. All of these performance parameters are related to an engine's ignition delay. Both fuel consumption and particulate emissions can be minimized by increasing the ignition delay up to a certain point where the engine structure cannot tolerate the rapid pressure rise. However, as the ignition delay period increases, the premix combustion rate increases, and a greater quantity of premixed fuel is present in the combustion chamber when ignition occurs. The larger quantity of fuel being ignited increases combustion pressure, combustion noise level, and NO.sub.x emissions. Therefore, with existing diesel engine designs, a trade-off exists between reduction of fuel consumption and particulate emissions on one hand, and reduction of noise and NO.sub.x emissions on the other hand.
In recent years, several methods have been proposed for reducing diesel engine ignition delay and premix combustion rate. These methods include dual (or pilot) fuel injection, fumigation, vigom fuel injection and staged fuel injection. See The Oil Engine and Gas Turbine, New Developments by B.I.C.E.R.A. (Nov. 1953) (discussing dual or pilot fuel injection); E. F. Obert, Internal Combustion Engines and Air Pollution (Interet Educational Publisher 3d Ed. 1973) (discussing fumigation); Eyzat, et. al., The Effect of the Vigom Process on the Combustion in Diesel Engines, S.A.E. Paper (October, 1964) (discussing vigom fuel injection); and Baker, Staged Direct Injection Diesel Engine, U.S. Pat. No. 4,543,930 (discussing staged fuel injection). However, neither dual fuel injection, fumigation, nor vigom fuel injection have been successful in reducing premix combustion rate without sacrificing fuel consumption and exhaust emission characteristics. The staged fuel injection method, however, has been successful to a limited degree in accomplishing these objectives.
Consequently, a need exists for an improved method and apparatus for operating a diesel engine that will simultaneously reduce fuel consumption, combustion noise, and NO.sub.x and particulate emissions.